Aqueous polyurethane dispersions are widely used as binders for aqueous paints. Among these paints, one pack systems are known which dry physically by evaporation of water, the film formation optionally being supported by addition of thermally activated crosslinking agents such as melamine formaldehyde crosslinker resins or blocked isocyanates which initiate curing at temperatures above 100° C.; and also two pack systems, where hydroxy-functional binder constituents are mixed with nonblocked isocyanates immediately prior to application, and the crosslinking reaction takes place at room temperature (or at a slightly elevated temperature in order to accelerate the cure).
Physically drying polyurethane dispersions are obtained, conventionally, by chain extending an isocyanate-functional prepolymer in the aqueous phase by means of polyfunctional compounds of low molar mass dissolved in water, referred to as chain extenders, such as diamines or dihydrazides which react with the isocyanate groups of the prepolymer.
Coatings obtained from aqueous systems comprising high molar mass polyurethanes with a mass average molar mass in excess of 20 kg/mol, measured as polystyrene equivalent mass, have been disclosed in U.S. Pat. No. 7,157,519 B2. Preferred polyols used therein are polyether polyols and polyester polyols. According to EP 1 493 950 B1, hardness, chemical and solvent resistance can be improved when choosing a polyurethane having a specific amount of substance of hydroxyl groups, —OH, of at least 0.6 mol/kg, a degree of branching measured as specific amount of substance of tertiary and/or quaternary aliphatic carbon atoms of from 0.01 mol/kg to 0.5 mol/kg, wherein the tertiary and quaternary carbon atoms are derived from trivalent and tetravalent alcohols, and additionally at least one of a specific amount of urea groups >N—CO—N< (formula I) in the polyurethane of from 0.8 mol/kg to 2 mol/kg, and a specific amount of substance of hydroxyl groups, —OH, of from 1 mol/kg to 4 mol/kg.
Although very good mechanical, and chemical resistance properties have been realised, adhesion of the coating to substrates may still be improved. It was therefore the object of this invention to provide a coating composition based on an aqueously dispersed polyurethane system that leads to coating films having good mechanical and chemical resistance, with improved adhesion on substrates coated therewith, particularly on plastic substrates.